1. Field of the Invention
The present invention relates to a cascode circuit used at a millimeter-wave band.
2. Description of the Related Art
In recent years, the range of application using radio waves at the millimeter-wave band such as a 60 GHz band wireless personal area network (WPAN) and a 76 GHz band millimeter-wave radar is widened. In association with this, a millimeter-wave device is required to have high gain and high output.
A generally known method to improve power gain is to cascode-connect transistors. Cascode connection is to connect to a drain of a source-grounded transistor a gate-grounded transistor. A circuit formed in this way is referred to as a cascode circuit.
A typical conventional cascode circuit is described in the following with reference to the attached drawings.
FIG. 24 is a circuit diagram illustrating a conventional cascode circuit.
In FIG. 24, a drain of a first transistor 51 a source of which is grounded is connected to a source of a second transistor 52 a gate of which is grounded. In order to ground a high-frequency signal, a gate of the second transistor 52 is grounded through a MIM capacitor 53. A gate of the first transistor 51 is connected to an input terminal while a drain of the second transistor 52 is connected to an output terminal.
It is to be noted that, although the cascode circuit illustrated in FIG. 24 is described in the context that high electron mobility transistors (HEMTs) are used as the transistors, the same can be said in a case in which hetero-junction bipolar transistors (HBTs) or the like are used. In this case, a base-grounded transistor is connected to a collector of an emitter-grounded transistor. In the following, a drain, a gate, and a source of an HEMT shall be able to be replaced by a collector, a base, and an emitter of an HBT, respectively.
As described above, a high-frequency signal at the gate of the second transistor 52 is grounded through the MIM capacitor 53. However, at the millimeter-wave band, the inductance of wiring connected to the MIM capacitor 53 and a parasitic inductance of a via hole can not be neglected. Therefore, a high-frequency signal at a desired frequency is short-circuited through a parasitic component. Therefore, there is a problem that, at the millimeter-wave band, even if the transistors are cascode-connected, the gain can not be satisfactorily improved.
Here, as an example, frequency characteristics of a maximum available gain (MAG) in a single emitter-grounded HBT and in a cascode-connected HBT are illustrated in FIG. 25.
With reference to FIG. 25, for example, at a microwave band of 10 GHz or the like, by cascode-connecting HBTs, the power gain is larger than that of the single HBT by about 10 dB. However, at the millimeter-wave band, as the frequency increases, the difference between the power gain of the cascode-connected HBT and the power gain of the single HBT becomes smaller. In particular, at a high-frequency band such as a 60 GHz band, a 76 GHz band, or the like, even a cascode circuit can not obtain a satisfactory gain.
It is to be noted that another method of improving the power gain is to continuously connect single transistors in series to increase the gain. However, this method has a problem that, as the number of the transistors and the number of peripheral circuits increase, the chip area increases and the cost increases as well.
In order to solve the above-mentioned problems, for example, the following can be referred to.
In a cascode circuit described in Japanese Patent Application Laid-open No. 2002-359530, as illustrated in FIG. 26, a first transistor 51 and a second transistor 52 are cascode-connected, and an open stub 54 having a length of about ¼ the wavelength of a signal at an operating frequency is connected to a gate of the second transistor 52.
Here, because, at the operating frequency, the gate of the second transistor 52 is grounded at a high frequency by the open stub 54, compared with a case in which a MIM capacitor and a via hole are formed in the vicinity of the gate and grounding is carried out, the parasitic component has less influence, and thus, satisfactory grounding is enabled.
Therefore, at the operating frequency, compared with a case in which the grounding is carried out using a MIM capacitor and a via hole, the power gain can be improved.
However, the conventional art has the following problems.
In the conventional cascode circuit disclosed in Japanese Patent Application Laid-open No. 2002-359530, a reflection gain is caused on an output side.
Here, as an example, FIG. 27 illustrates frequency characteristics of a reflection gain/loss on an output side in a single HEMT, a cascode circuit in which a gate of a second transistor is grounded through a MIM capacitor (see FIG. 24), and a cascode circuit disclosed in Japanese Patent Application Laid-open No. 2002-359530 in which a gate of a second transistor is grounded through an open stub (see FIG. 26).
In FIG. 27, the cascode circuit grounded through a MIM capacitor has a reflection gain at a frequency band of about 20-90 GHz. The cascode circuit grounded through an open stub has a reflection gain at a frequency band of about 70 GHz or more.
When the cascode circuit has a reflection gain, unnecessary oscillation is caused. If the cascode circuit is applied to, for example, an amplifier, there is a problem that stable normal operation may not be expected.
Further, if the cascode circuit is applied to, for example, an oscillator, there is a problem that satisfactory output may not be expected.